Antimicrobial compounds, such as antibiotics and drugs, have been used as a part of dairy cattle management for several decades. Unfortunately, antimicrobial compounds used in treatments may enter the milk supply. Once present in milk, they are difficult to eliminate, and their presence is of public health concern.
An estimated 5 to 10% of American adults are hypersensitive to antibiotics. After ingestion, as little as 0.003 IU of penicillin G may cause allergic responses. The antibiotic residues also may create an environment that is favorable for resistant bacterial strains. The presence of antibiotics in milk has .also created problems in the dairy industry, including inadequate curdling of milk, improper ripening of cheeses, and decreased acid or flavor production. A 50% inhibition of cheese and yogurt starter cultures by 0.2 IU/ml of penicillin was reported. The FDA considers antibiotic-contaminated milk to be adulterated. Therefore, the test of antimicrobial compounds, such as antibiotics, in milk is important for the dairy industry.
The sensitivities of several currently used methods for the detection of penicillin G in milk are the Sarcinia lutea cylinder-plate method, 0.02 IU/ml; Penzyme test, 0.01 IU/ml; Bacillus stearothermophilus disc assay, 0.005 IU/ml; Charm test, 0.005 IU/ml; and Delvotest-P, 0.005 IU/ml. For the detection of trace residues (&lt;0.005 IU/ml) of penicillin G in milk, a more sensitive, and preferably automatic, method is needed.
The changes in electrical properties of the culture media have been utilized for the rapid estimation of total, mesophilic, and psychrotrophic counts, coliforms, salmonella, abnormal milk, and bacteriophage in Cheddar cheese making. Electrical change was also used as a growth index of lactic acid bacteria in milk.
Okigbo and Richardson reported an impedance method using a Bactometer 123 impedance instrument to detect streptomycin or penicillin G in sterile milk inoculated with 5% active or 5% inactive lactic culture. Okigbo, O. N., and G. H. Richardson (1985), Detection of penicillin and streptomycin in milk by impedance microbiology, J. Food Protection, 48:979, which is hereby incorporated by reference. Although the test is more sensitive (0.001 IU/ml) than previously reported tests, quantitative results were not obtained.